1. Field of the Invention
The present invention relates to a voltage boosting circuit using a chopper circuit, and an electrically-driven power steering device using the voltage boosting circuit.
2. Description of Related Art
An electrically-driven power steering device has advantages that no hydraulic system is used, a structure for assisting the steering control force through motor driving is simplified, the fuel consumption can be improved because the motor is driven only when it is required, and the steering control force can be made heavier or lighter in accordance with the vehicle speed.
In the electrically-driven steering device as described above, the power supply voltage of an in-vehicle mount battery is supplied to the motor through an electrical control circuit, and the magnitude of the current is controlled in accordance with the driving state of the vehicle. Specifically, the electrical control circuit detects steering (steering speed, steering torque) and outputs a proper control signal by referring to a preset steering sensitive characteristic (a characteristic for controlling the magnitude of supply power in accordance with steering), and detects a vehicle speed and outputs a proper control signal by referring to a preset vehicle speed sensitive characteristic (a characteristic for controlling the magnitude of supply power in accordance with the vehicle speed). The electrically-driven power steering device reduces the steering assist force (steering assisting force) in connection with increase of the vehicle speed, or increases the steering assist force in connection with increase of the steering speed or steering torque.
Current flowing in the electric control circuit of the electrically-driven power steering is normally varied in the range from several amperes to several tens amperes in accordance with the magnitude of the load at the auxiliary assist time, and there is a case where voltage drop caused by a current control element (for example, a switching device such as MOSFET or the like) or a wire cable cannot be neglected when large current is supplied to the motor, for example, when a steering wheel is turned back at the parking time, a vehicle runs at a super-slow speed or the like. That is, this voltage drop remarkably reduces the current supply efficiency with the voltage drop of the input voltage of the motor, and also it is impossible to supply necessary current to the motor, so that the steering assist cannot be sufficiently performed.
Therefore, JP-A-8-127350 (Patent Document 1) has proposed a device in which the power supply voltage is boosted by a chopper circuit and which is equipped with means for controlling the magnitude of the voltage to be boosted in accordance with the vehicle speed or steering.
Furthermore, Japanese Patent No. 3205651 (Patent Document 2) has proposed a device in which a double voltage power supply circuit including two switching devices and two capacitors connected in a bridge style is used, and the ON/OFF duty ratio of the switching devices is changed in accordance with the vehicle speed or steering to control the magnitude of the output voltage to a predetermined voltage higher than the power supply voltage.
Still furthermore, JP-A-2005-51951 (Patent Document 3) has proposed a device for disconnecting an inductor and a power source from each other and carrying out a predetermined failure judgment when short-circuiting failure or open failure judgment of a switching device is carried out in a voltage boosting circuit using a chopper circuit.
However, in the device disclosed in the Patent Document 1, when the switching device of the chopper circuit suffers closed-circuit failure, it falls into an output short-circuit state, so that the output of the power source is not supplied to the motor side.
With respect to the chopper circuit used in the electrically-driven power steering device, current of several amperes to several tens amperes flows as described above, and thus even when a switching device having a high withstanding voltage (power MOSFET or the like) is used, the chopper circuit may be broken in accordance with permissible withstanding quantity of the switching device when there occurs a situation that large current of about several tens amperes flows for a long term. When the switching device of the chopper circuit suffers short-circuiting failure and thus the output of the power source is not supplied to the motor side, the steering assist force is suddenly reduced to zero, so that the steering operation becomes very heavy suddenly. For example, if the switching device suffers short-circuiting failure when a vehicle starts its movement from the stop state under the state that it is located at a turn-right lane before an intersection, the steering operation to the right side becomes remarkably heavy, so that it may be impossible to sufficiently carry out the steering operation.
The Patent Document 2 is different from the Patent Document 1 in that a bridge-connection switching circuit is constructed. However, even with this circuit construction, any one of two switching devices may have a breakdown by large current, and if it has a breakdown, the power source output is prevented from being supplied to the motor side by a safety circuit.
The Patent Document 3 discloses that the short-circuiting or open failure judgment of the switching device of the chopper circuit is efficiently carried out. However, it is considered that under the failure the power source output is not supplied to the motor side.